Volume 34, Issue 11

Weeds and their control were mentioned already in the Bible. For hundreds of years weed control was performed by hand, with tools and machines. Since more than thirty years “Chem­istry” has been helping out to a steadily increasing degree. More than 100 different active sub­stances are available for this purpose today. The use of herbicides has grown at this tremendous rate because many of the people formerly employed in “Farming” discovered that other sectors of the economy offered better opportunities for earning an income with shorter hours and more pleasant working conditions; so they gradually left agriculture. In view of the lade of any other solution farmers had to use chemistry to substitute for the absence of sufficient manpower. A reversal of this development is hardly to be expected in spite of the growing popularity of the environmentalists’ slogan “Bade to Nature”. For one thing, the rationalization that was more or less forced on agriculture finally helped the farmer approach the high standard of living enjoyed by the rest of society. Agriculture without chemical| weed control would only become plausible if the people working in agriculture were again willing to accept longer workdays and greater discomfort (dust, dirt, wet and cold) and to agree voluntarily to waive a portion of their potential income. The resultant unit production costs in that case would mean that the consumer would have to also be willing to spend more for food and thus cut back the share spent on other items related to a high quality of living, such as leisure time, holidays and cultural activities. As the realization of these preconditions sharply contradicts the progressive nature of man and the trends of history, herbicides will maintain their position in agriculture. However, we are all called upon to integrate the use of chemical weed control agents with other cultural practices available in an economically and biologically responsible manner. Successful basic research must remain the foundation of social progress, and this in turn must create the financial prerequisites for flourishing basic research.

Herbicide Selectivity, Photosynthesis Inhibitors, Accumulation, Inactivation, Chloroplast Sensitivity Various sources are involved in the selective action of herbicides which inhibit photosynthesis, ranging from gross morphological differences connected with the availability of the herbicide to the foliar or root surface, to differential absorption, translocation and inactivation in the tissues of various plants species, and finally to differences in susceptibility at the chloroplast level. Their significance is reviewed with respect to the selectivity in the field.

The homogeneity, density and axenic nature of cell culture system s have made them an amenable tool for studying many aspects of biocide research; screening, metabolism , m ode of action and resistance. Cell cultures are “multi-homogeneous” and each homogeneous state may be analogous to different plant parts. Cultures can be homogeneously green, homogeneously white in either a homogeneously exponential or stationary phase of growth. Examples are be presented showing why no single homogeneous state should be envisaged to be analogous to a whole plant in biocide studies. Because of the possibility of more uniform herbicide application, culture systems have a use in isolatin g resistant crop strains, at higher selection pressures and much higher “plant” densities than is possible in the field, with recent successes. Protoplast fusion techniques may allow the transfer of genetic resistance between related but genetically incompatible crop species.

Metabolism of chloridazon, the active ingredient of the herbicide Pyramin, was studied in cell suspension cultures of Beta vulgaris (sugar beet), Papaver bracteatum (poppy), Phaseolus aureus (mung bean), Glycine max (soya bean), Nicotiana tabacum (tobacco), and Petroselinum hortense (parsley). Metabolism of the analgetical and antipyretical drug antipyrin (phenazon) was also investigated. In Beta vulgaris and in Papaver bracteatum small amounts of both compounds are hydroxylated, the main quantity remaining unmetabolized. In Beta vulgaris chloridazon is converted to p-hydroxy- chloridazon, whereas antipyrin yields three different compounds, namely p-hydroxyantipyrin, hydroxymethylantipyrin and a further hydroxyantipyrin, which was identified as o- or m-hydroxy- antipyrin. Cell suspension cultures of Papaver bracteatum hydroxylate chloridazon to p- and m- hydroxychloridazon, respectively, and antipyrin to p-hydroxyantipyrin and hydroxymethylantipyrin respectively. The metabolites, with exception of o- or m-hydroxyantipyrin, were identified by spectroscopic methods. The structure of the latter metabolite was established by comparing Up­values of the metabolite and the compound synthetically available. Metabolism in sugar beet and poppy cultures is similar to metabolism in mammals rather than in bacteria. Cell suspensions of chloridazon-insensitive sugar beet and of chloridazon-sensitive poppy are severely inhibited in growth by chloridazon, whereas antipyrin, even when applied in high concentrations, does not induce growth inhibition. In soya bean, mung bean, tobacco, and parsley neither chloridazon nor antipyrin were found to be metabolized.

Under field conditions atrazine inhibits CO 2 fixation but the electron flow seems to be hindered partly only. The remaining capacity of the electron transport chain produces not used reducing electrons. The consequence should be a destructiv photooxidation. This process however is suppressed by photorespiration. When all possible substrats including stored carbohydrates are metabolized by photorespiration photooxidation is not hindered any longer and destroys the plant.

The different sensitivity of broad-leave weeds against the photosynthetic inhibitors phenmedipham and bentazon could be explained by the combined action of herbicide uptake and decomposition. In spite of fast uptake of phenmedipham by Beta vulgaris, this plant was not injured, because the herbicide was metabolized very rapidly. The transformation of phenmedipham by Galium aparine, Matricaria chamomilla, Centaurea cyanus, Stellaria media and Amaranthus retroflexus was reduced compared to Beta vulgaris. These plants survived, with the exception of Amaranthus, first of all because of a reduced uptake of the herbicide. The other weeds investigated (Galinsoga ciliala, G. parviflora, Sinapis alba, Capsella bursa-pastoris, Sinapis arvensis, Senecio vulgaris) were extremely sensitive against phenmedipham because they could not protect themselves neither by a reduced uptake nor an increased metabolism. The grass weeds Alopecurus myosuroides and Avena fatua are resistant against the herbicide because of the protected growing point and the missing basipetal translocation of the compound. Also the susceptibility of rice, soybeans, wheat and some weeds to bentazon is the result of penetration and metabolism of the herbicide in plants. Susceptible plants (Chrysanthemum segetum, Matricaria chamomilla, Xanlhium pensilvanicum) are nearly unable to detoxify bentazon. whereas in resistant plants (rice, soybeans, wheat) a substantial amount of absorbed bentazon was converted to different metabolites. Only Echinochloa crus-galli, which is more resistant, and Galium aparine, which is more susceptible, when they adsorbed the same quantity of bentazon, deviate from this general pattern.

While searching for chemical compounds with herbicidal or growth regulating activities the researcher gladly notices if intact plants respond to the treatments at all. By means of colour slides, which unfortunately could not be printed here, changes in plant growth and leaf colour induced by chemicals were demonstrated as they usually occur in an industrial screening operation. The range of symptoms shown in the presentation included simple lethal necrosis, “bleaching”, increased generation of anthocyans, deterioration of the plant tissue, control of axillary buds, the prevention of resprouting, stunting of the shoots, and epinasties. Chemical compounds synthesised in the BASF laboratories and commercial products were applied.

Among the adaptory responses of plants to different ecological conditions the adaptation to low light intensities is one of the most important. This response, known as “shade adaptation”, may, however, be similarly induced by decreasing the rate of photosynthesis in other ways. The partial inhibition of photosynthetic electron flow with herbicides has clearly been shown to induce shade- type plants; in the case of photosynthetic limitation by decreased CO 2 -availability only certain aspects of the shade-type metabolism - namely the changes occurring in the nitrogen metabolism - have been demonstrated.

The influence of two biocides (bentazone, triadimefon) on the growth-response of radish seedlings (Raphanus sativus L.) was investigated with special emphasis on the development of sun or shade-type chloroplasts. 1. The fungicid e triadimefon causes strong-light growth-response and the formation of sun-type chloroplasts as seen from the changed chlorophyll a/b ratio and the carotenoid and prenyl-quinone com position. 2. The photosystem II herbicide bentazone, in turn, induces a shade-type adaption. It prevents the strong-light induced formation of sun-type chloroplasts, but has little influence on the chloroplast com position in weak light.

The influence of photosystem II herbicides (DCMU, bentazone) and herbicides that block chloroplast-pigment formation (amitrol, SA N 6706) on the incorporation of [ 14 C] acetate and [ 3 H ]- mevalonate in chlorophylls and carotenoids was studied in radish seedlings (Raphanus sativus L .). 1. Though the pigment content in SAN 6706-treated plants is very low, they exhibit the highest specific incorporation rate as compared to controls and to the other herbicide-treated plants. This high metabolic activity coincides with a high specific activity of HMG-CoA-reductase in the plastidic and cytoplasmic fraction. 2. [2- 3 H] mevalonic acid was incorporated in chlorophylls and carotenoids of controls and all herbicide-treated plants. 3. [2- 14 C] acetate was only incorporated in β-carotene and lycopene, but not in α-carotene or xanthophylls. From this it is concluded that β-carotene is synthesized by two different biosynthetic pathways. 4. The incorporation of [ 14 C ]acetate in DCMU- and bentazone-treated plants proceeded at a much lower rate (chlorophyll a) - or not at all (chlorophyll b) - than in controls of plants treated with amitrol or SAN 6706.

Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4 (3H)-one 2,2-dioxide] causes an increase in the chlorophyll fluorescence in the leaves of mustard plants [Smapis alba (L.) ] as a result of its in­hibitory effect on the photosynthetic electron transport. The rate of this fluorescence induction depends on the pH of the suspension medium. As the concentration of hydrogen ions diminishes, the maximum fluorescence that indicates a total inhibition of photosynthesis is reached at a later time. In the case of single cells isolated from the leaves of soybean plants [Glycine max (L.) Mer “Harosoy”], bentazon inhibits photosynthetic CO 1 , assimilation. The amount of inhibition is depen­dent on the pH of the suspension medium. The uptake of CO 2 is inhibited more at pH 6 than at pH 7 or 8. Bentazon is more readily absorbed by cells at pH 6 than at pH 7 or 8. The inhibition of CO 2 assimilation is proportional to the amount of bentazon taken up.

The enzymatic activity from sugar beet leaves which is responsible for the detoxification of the herbicide metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1, 2, 4-triazin-5-one, trade name Goltix ® ) has been characterized in vitro. The detoxification occurs by rapid deamination in vivo as well as in vitro. However, the deamination in vitro is only maximal under reducing conditions, i. e. with an electron donor and in a nitrogen atmosphere. The electron donor may be cystein, glutathione, dithionite or ascorbate. The enzymatic deamination further requires the addition of cytochrome c and a “supernatant factor”, which may be replaced by FMN, FAD or DCPIP. However, in the presence of FMN or DCPIP cytochrome c is not essential but only stimulatory. The partic­ulate as well as the soluble metamitron deaminating enzyme preparations obtained take up oxygen when supplied with cysteine and FMN. The particulate enzyme appears in the peroxysome-fraction. It is therefore suggested, that the enzymatic deamination of metamitron in sugar beet leaves is mediated by a proxisomal membrane bound electron transport system which alternatively may reduce oxygen or metamitron (deaminating).

Inhibition of photosynthetic O 2 evolution of algal cells by phenol herbicides (e. g. ioxynil, di-iodonitrophenol, dinitrothymol) and dibromothymoquinone is reversed during treatment within one or two hours. Addition of the same or related inhibitory phenolic compounds as well as of dibromothymoquinone in higher concentrations to these pretreated algae causes no inhibitory effect. The cells become temporarily resistant to these inhibitors. Algal cells pretreated with phenol herbicides are still sensitive to diuron and representatives of some other groups of herbicides. After treatment with dibromothymoquinone, however, the sensitivity of cells to diuron is lowered.

The algal excretion product geranylacetone was proved to be an effective and novel inhibitor of carotene synthesis in Synechoccus 6911. The application of geranylacetone resulted in an accumulation of phytofluene, indicating an inhibition of the conversion of this compound into ξ-carotene. The cis-isomer of geranylacetone was as effective as the natural trans-isomer. The chloro­phyll synthesis was only slightly affected.

Short term effects of 2-(C-dichloro-acetylamino)-3-chloro-1,4-naphthoquinone (Hoe 13465, quinonamid*) and 2-amino-3-chloro-1,4-naphthoquinone (Hoe 17399, 06K-quinone) on cell suspensions of Chlorella vulgaris, Anabaena flos aquae, Porphyridium cruentum, and on isolated spinach chloroplasts were studied. The results clearly show that both substances inhibit the photosynthetic O 2 production of algal suspensions as well as the electron transport of PS II in spinach chloroplasts. PS I is not inhibited by the action of the two algicides. At low concentrations quinonamid acts as a photosynthetic electron transport blocker, whereas Hoe 17399 is a weak inhibitor of photosynthetic electron flow. Mode of action of the two naphthoquinones is discussed. Both naphthoquinone derivatives can operate as an electron acceptor for PS I at low concentra­tions (10 -5 -10 -6 м). In addition there is observed a strong stimulation of dark respiration in algal cells induced by both of the compounds, Hoe 17399 causes a much higher stimulation rate than quinonamid does.

Bioassay techniques and procedures for the determination of photosynthetic inhibitors in situ , in vivo, and in vitro are reviewed. Examples of bioassays for herbicide development and research are given.

This paper reviews the inhibitory effects of herbicides at three locations within photosynthetic electron transport, on photophosphorylation, and on pigment synthesis

Relative activities and sensitivities towards twelve 1,2,4-triazinones were determined by conventional dilution techniques. The pI 50 -values of photosynthesis inhibition were found in a very narrow range (6.4 -6.6) whereas their physical properties differed widely (R M , log P O/W * log P SE , log (1/c) W). The question was whether the physical parameters showed any correlation with the relative activities and relative sensitivities which differed very widely (up to 50 fold). It was found that only the water solubility (log (1/c) W) showed some correlation with activity and no correlation could be detected between the chemical structures and the activities or sensitivities.

Carotenoids are chloroplast constituents. In vivo different carotenoids may be present at different sites within the chloroplast, and may have different functions. Especially important is a role in protecting the chloroplast and hence the plant from chlorophyll-sensitized photooxidation. If carotenoid biosynthesis can be blocked by herbicides, such protection will no longer be afforded, and rapid death of the plant is likely. In this article current knowledge of carotenoid biosynthesis is reviewed. The main characteristics of each stage in the biosynthesis are outlined and the suit­ability of these reactions as possible herbicide targets is assessed. Two experimental approaches to the study of the effects of herbicides on carotenoid biosynthesis are suggested.

Alkyl substituted 2-halogen-4-nitro-phenols (like bromonitrothymol) are potent inhibitors of photosynthetic electron flow in chloroplasts. There inhibition site is identical to that of DCMU, though the phenols do not contain their essential chemical element responsible for inhibition nor do they follow their correlation of chemical structure and biochemical function. Binding of labelled metribuzin on the chloroplast membrane and its replacement by the phenol inhibitors support the notion of identical binding sites, however. In trypsin treated chloroplasts, the primary acceptor of photosystem II becomes accessible to ferricyanide, whose photoreduction is therefore DCMU insensitive. After trypsin treatment also the sensitivity of ferricyanide reduction to phenol inhibitors is lost as is the binding of labelled metri­buzin. The dinitrophenylethers of bromonitrothymol and of ioxynil and other diphenylethers are potent inhibitors of photosynthetic electron flow. Their site of inhibition is after the function of plastoquinone, i. e. they are DBMIB analogues.

Binding of the triazinone derivative metribuzin to thylakoid membranes was studied in depend­ence of temperature and trypsin pre-treatment of the chloroplasts. The temperature curve of the binding constant suggests that the lipid phase of the membrane contributes to inhibitor binding. Trypsin pre-treatment of the membranes removes the receptor sites for the inhibitor, while the trypsin effect on the affinity of the receptor to the inhibitor is marginal. Concomitantly trypsin treatment releases the inhibitory effect of metribuzin on the electron transport from DPC to DCPIP.

This report reviews studies which provide evidence defining the mode of action and site of action of photosystem II (PS II) herbicides; the involvement of the secondary electron carrier on the reducing side of PS II (called B) is indicated as the target site for these compounds. These studies of the action of PS II-inhibitors were performed in chloroplasts of various weed species in order to define the mechanism which is responsible for herbicide tolerance at the level of chloroplast membranes in newly discovered triazine-resistant weed biotypes. Many species of triazine-resistant weed biotypes have been collected in North America and Europe. Where data is available, these plants have been found to share the following common features: a) they were discovered in areas where triazine herbicides had been used repeatedly, b) resistance to the triazines is extreme; it is not due to a minor shift in herbicidal response, c) no changes in herbicide uptake, translocation or metabolism - as compared to susceptible biotypes - can be detected, d) resistance is selective for only certain classes of photosynthetic herbicides, and, e) chloroplasts isolated from triazine-resistant weeds display high preferential resistance to the triazines in assays of photosystem II partial reactions. To focus on the mechanism which regulates preferential herbicide activity, we have characterized susceptible and resistant chloroplasts in the presence and absence of herbicides. Properties of the PS II complex of chloroplasts from several different triazine-resistant weed biotypes share the following traits: a) the herbicide binding site (as measured by direct binding of radiolabeled herbicides or by inhibition experiments) is modified such that the affinity for triazines is dramatically reduced. b) alterations in response to many PS II-herbicides occur such that the triazine-resistant chloroplasts are very strongly resistant to all symmetrical triazines, strongly resistant to assymmetrical triazinones, partially resistant to pyridazones and uracils, only slightly resistant to ureas or amides, and increasingly susceptible to nitrophenols, phenols and the herbicide bentazon (all as compared to susceptible chloroplasts), c) there is a change in the reaction kinetics of the electron transport step between the primary and secondary electron acceptors (referred to as Q and B ), and d) in two examples, specific small changes in a membranepolypeptide can be detected in the resistant thylakoids. We suggest that certain amino acids or segments of the apoprotein of B (the bound quinone which acts as the secondary electron carrier) are modified or deleted in these chloroplasts. Such a polypeptide change could affect both the redox poising of the Q - /B reaction pair, and the specific binding of herbicides.

Based on the functional organization scheme of system-II-electron transport and its modification by different procedures a proteinaceous component enwrapping the redox components (plastoquinone molecules) of the acceptor side (thereby acting as regulatory element) is inferred to be the unique target for herbicidal interaction with system II. This proteinaceous component, which is attacked by trypsin, provides the receptor sites for the herbicides. Studies of the release kinetics in trypsinated chloroplasts of the inhibition of oxygen evolution with K 3 [Fe (CN) 6 ] as electron acceptor indicates, that there exists a binding area with different specific subreceptor sites rather than a unique binding site for the various types of inhibitors. Furthermore, trypsination of the proteinaceous component enhances the efficiency of the plastoquinone pool to act as a non-photochemical quencher for excitation energy.

With trypsin-treated spinach chloroplasts and the assay system H 2 O→ FeCy, photosynthetic electron transport inhibitors could be subdivided into four groups according to their different inhibitory behaviour under the conditions used. 1. Inhibition exerted by ureas, triazines, triazinones and pyridazinones was reversed to 50 - 60% as against the untreated control. 2. Inhibition by dibromothymoquinone, trifluralin or the 2,4-dinitrophenylether of 2-iodo-4-nitrothymol was almost completely reversed. 3. Inhibition caused by higher concentrations of nitrofen or the analog RH 2512 could only be reversed to a small extent. 4. Phenol herbicides showed increased inhibitory activity during short-term assays.

The herbicide 4,6-dinitro-o-cresol inhibits electron transport to ferricyanide and non-cyclic photophosphorylation for 50% at about 15 μm. At higher concentrations the photosystem I depen­dent Mehler reaction ascorbate/dichlorophenolindophenol to methyl viologen is stimulated, while cyclic photophosphorylation is inhibited. The herbicide thus is an inhibitory uncoupler. Although the chemical structure of 4,6-dinitro-o-cresol is different from that of the diuron-type herbicides, its site and mechanism of action is similar. Both 4,6-dinitro-o-cresol and diuron inhibit electron transport between the primary electron acceptor of Photosystem II and the plastoquinone pool. This causes a closing of the reaction centers of Photosystem II. The interaction with the inhibited molecule however is different for the two herbicides.

A number of 16 substituted diphenylamines have been tested for their inhibitory activity on photosynthetic NADP reduction and photophosphorylation. The most active compounds exhibited pI 50 values of 6.0 in photosynthetic electron transport and 6.8 in cyclic photophosphorylation, respectively. The inhibition site in electron flow of the diphenylamines is located between the two photosystems. Necessary for high activity is the substitution of the phenyl moieties by several strongly electron withdrawing substituents. A quantitative structure activity relationship according to a parabolic Hansch approach could be accomplished by using the Hammett electronic parameter as the only variable.

Substituted phenyl-hydroxy pyrrolone derivatives are potential herbicides and are found to be effective inhibitors of photosynthetic electron flow in chloroplasts. Their site of inhibition is localized at the acceptor side of photosystem II similar to DCMU. They have no effect on photophosphoryla­tion. The correlation of photosynthetic acitivity to chemical structure is studied by comparing 36 derivatives. A clear dependence of inhibitory activity on lipophilicity is found. The dependence of herbicidal activity on chemical structure is more complex.

The treatment of flax cotyledons (Linum usitatissimum) with paraquat was shown to decrease the levels of chlorophyll and carotenoid pigments. The fatty acid content of chloroplast fragments isolated from treated tissue was determined and shown to be greatly decreased by paraquat treat­ment. The superoxide radical was demonstrated to play an important role in the phytotoxic action of paraquat by the use of a copper chelate of D-penicillamine, which has a high superoxide dismutating activity. The action of paraquat was inhibited by this compound. The role of superoxide is discussed with reference to the generation of more toxic species, such as singlet oxygen.

The chlorosis induced by several different herbicides (group 1: aminotriazole, haloxidine; group 2: Sandoz 6706, difunon) in developing leaves of rye seedlings (Secale cereale L.) growing in light of 5,000 lx was, in addition to the absence of chlorophyll and carotenoids, accompanied by specific deficiencies of the 70 S ribosomes and of both chloroplastic and peroxisomal enzymes as previously described (Plant Physiol. 61, 1017 - 1022 (1978)) while growth and mitochondrial activities were little or not impaired. In dim light (10 lx) chlorophyll was formed and the chloroplastic and peroxisomal enzymes reached activities comparable to those in untreated leaves at 10 lx. Upon exposure to 30,000 lx a rapid bleaching of existing chlorophyll and a rapid inactivation of the plastidic enzyme NADP-glyceraldehydephosphate dehydrogenase occured after treatment with herbicides of group 2 but not in the presence of herbicides of group 1. After all treatments catalase was rapidly inactivated after transfer to high light intensity. The bleaching of chlorophyll and the enzyme inactivations occurred also at 0 °C. In the herbicide-bleached leaves grown at 5,000 lx the in vivo synthesis of (δ-aminolevulinic acid was low and in the presence of Sandoz 6706 and difunon the capacity for protochlorophyll (ide) synthesis was largely inactivated.

Light dependent chlorophyll bleaching and ethane formation in Euglena gracilis cells at slightly acidic pH values above 5.2 are enhanced by DCMU and other photosystem II blockers. Excited chlorophyll under conditions of limited energy dissipation seems to be the oxidant for ω-3 unsaturated fatty acids, yielding ethane and other (ethylene, “malondialdehyde”) oxidation products, cooxidizing chlorophyll.

Two strains of Scenedesmus acutus were found useful to study the influence of bleaching agents on either the greening process or the fully pigmented algal cell during growth. Both physiological conditions exhibit high sensitivity to bleaching herbicides. With this new assay, contrasting bleach­ing effects with the same compound can be found allowing differentiation of the herbicidal action of bleaching agents which apparently is a multifunctional one. Furthermore, the I 50 can be determined rather rapidly in a simple graphical method by a Dixon plot. A subsequent application of bleaching herbicides to cultures of the fungus Phycomyces blakesleeanus rules out a possible specific action of the compounds assayed on chlorophyll or photosynthetic redox carriers. This latter assay can show whether or not the herbicides synthesis as is the case with difunon or SAN 9789.

Over 24 and 48 hour cultivation periods the influence of SAN 9789 (norflurazon), EMD-IT 5914 (difunon) and fluridone on growth, photosynthetic oxygen evolution and pigment content of the green alga Scenedesmus acutus was determined. Four effects were observed: a) Both carotenoid and chlorophyll formation were inhibited. b) Carotenoids were destroyed in the presence of air, but not nitrogen. The level of chlorophyll, however, did not change. c) β- (and α-) carotene was markedly decreased in the presence of oxygen. d) Photosynthetic oxygen evolution was decreased with the disappearance of carotenoids. These effects, which are accompanied by reduced growth, are believed to represent primary herbicidal modes of action. The decrease of oxygen evolution is not due to a direct inhibition of photosynthetic electron transport by the herbicides applied.

The pyridazinone herbicides influence to several degrees, the Hill reaction, the pigment formation, the ratio of saturated to unsaturated fatty acids and the chloroplast ribosomes. Which is the primary effect is not yet known.

Treatment of wheat caryops with 3-amino-1,2,4-triazol followed by dark germination leads to phytol deficiency (based on the chlorophyll content) during the subsequent greening process in the light. The analysis of the pigment fraction reveales the presence of chlorophyll esterified with geranylgeraniol and dihydrogeranylgeraniol in agreement with earlier results [5]. Chlorophyll esterified with geranylgeraniol is shown in vivo and in vitro to be the precursor of phytylated chlorophyll. Aminotriazol presumably inhibits the hydrogenation of geranylgeraniol to phytol in the pigment.

Monuron treatment of detached flax cotyledons caused a rapid inhibition of CO 2 fixation in darkness and light. Chlorophyll breakdown was promoted by increased light intensity. Ethane generation, an indicator of membrane damage, increased as the chlorophyll level decreased. The carotene pigments were destroyed more rapidly than the xanthophylls and chlorophylls. Treatment of cotyledons with monuron in the absence of oxygen almost prevented carotenoid and chlorophyll loss. The addition of the singlet oxygen quencher DABCO delayed chlorophyll breakdown. The results are discussed in relation to a role for singlet oxygen in the herbicidal action of monuron.

Effects of the chlorinated hydrocarbon insecticide Lindane on membrane transport of two neutrale amino acids in the Cyanobacterium Anacystis nidulans (Synechococcus AN) were measured. In white light the ʟ-Leucine incorporation into the protein fraction was inhibited, increasing with time. After 30 minutes the degree of inhibition was the same as the effect of DCMU (5×10 -6 m) on ʟ-Leucine incorporation. 14 CO 2 -Rxation was also reduced at this time. At 717 nm, which en­ables PSI activity allone, no inhibition was observed. The light energy dependent membrane transport itself of ʟ-leucine in presence of CAM and of the non-metabolisable α-AIB in white light and in monochromatic light of 630 nm and of 717 nm were not influenced by Lindane. The different sites of Lindane action are discussed. It is assumed that in 30 minutes chiefly photo­synthesis (PS II and CO 2 -fixation) is affected by Lindane, resulting in a suppression of protein synthesis caused by a depletion of intermediates of CO 2 -fixation.

Microbial Metabolization of Hexachlorocyclohexane, Anaerobic and Aerobic Microorganisms, Dechlorination of Hexachlorocyclohexane Microbial degradation of lindane or other hexachlorocyclohexane isomers has been observed to occur more rapidly in flooded than in upland soils. Experiments with a mixed bacterial flora enrich­ ed anaerobically from soils showed a rapid dechlorination and degradation of lindane. Screening studies with pure bacteria indicated that Clostridium spp. and several representatives of Bacillus spp. and Enterobacteriaceac effectively degraded lindane, while representatives of Lactobacillaceae and Propionibacterium were inactive. During degradation organic bound chlorine was released as chloride and nearly chlorine-free, partly volatile metabolites were formed. The chloride formation was promoted by the addition of glucose, pyruvate or formate. Lindane was the most easily degraded isomer, while α-and especially β-and δ-hexachlorocyclo-hexane was also, but more slowly, dechlorinated. These isomers are relatively frequently observed as environmental contaminants, their occurrence might be caused by their greater persistence or / and by an isomerization of the γ-into α-and other isomers during microbial incubation as it was discussed by several authors. Experiments, however, whether or not this isomerization can be carried out by microbes, were negative or showed that it only can occur to a very minor extent. Aerobic degradation of lindane proceeded only slowly and chiefly by formation of chlorinated aromatic compounds. One of the main intermediate metabolites seemed to be γ-pentachlorocyclo-hexene which was further transformed by the release of HCl or hydrogen and by addition of water. During anaerobic incubation, γ-tetrachlorocyclohexene intermediately occurred which was further dechlorinated to nearly chlorine-free compounds.

The inhibitory effects of the insecticides Allethrin, Lindane, and Jacutin-Fogetten sublimate on photosynthetic electron transport of broken chloroplasts were tested. 50 μmol l -1 Allethrin caused an inhibition of 80% of the benzoquinone and ferricyanide Hill-reactions. 39 μmol l -1 Lindane inhibited the basal, coupled and uncoupled electron transport to ferricyanide up to 35%. The precipitate formed by the sublimation of Jauctin-Fogetten containing Lindane depressed electron transport much more than pure Lindane. 50 μg ml -1 of the sublimate led to an 80% inhibition of ferricyanide Hill-reaction.

Side effects of the chlorinated hydrocarbon insecticide Lindane on the plasma membrane of the submerged macrophytic fresh-water plant Elodea densa were studied. Single glass microelectrodes were inserted into single cells and membrane potential, membrane resistance and electrode resistance were recorded using a new designed electrophysiological monitoring system (“ELM 2”). The current-voltage relationship in the plasma membrane of Elodea cells under passive conditions is nearly linear for relaxing current pulses up to 10 -8 amperes. Lindane treatment, produced first (3.5 hours) a strong nonlinearity in current-voltage relationship with increased membrane resistance for inward current flow, and later (24.5 hours) an increased membrane resistance both for inward and outward current flow. It is discussed that the earlier reported lowering of the potassium selectivity in the plasma membrane of Elodea after Lindane treatment will be the reason for the alterations of current-voltage relationship.